1. Field of the Invention
The present invention relates to aluminum (hereinafter simply referred to Al) alloy excellent in anti-corrosiveness to gas and plasma and particularly, to Al alloy suitable for a structural material to build an apparatus, in which gas or plasma including a corrosive component and an element is used, such as a production apparatus for semiconductor or liquid crystal.
2. Prior Art
A production apparatus for semiconductor or liquid crystal such as a chemical or physical vapor deposition apparatus, that is CVD or PVD, or a dry etching apparatus is constructed of a heater block, a chamber, liner, a vacuum chuck, an electrostatic chuck, a clamper, bellows, a bellows cover, a susceptor, a gas diffusion plate, and an electrode etc. as main constituents. In the interior of such a production apparatus for semiconductor or liquid crystal, since a corrosive gas, as reaction gas, including a halogen element such as Cl, F, Br and/or the like, and/or elements such as O,N, H, B, S, C and/or the like is introduced, the constituent members are required to have anti-corrosiveness to the corrosive gas. Furthermore, the main constituent members are necessary to have anti-corrosiveness to plasma since halogen containing plasma is also generated in the interior of the production apparatus in addition to presence of the corrosive gas.
Conventionally stainless steel has been used for a structural material of such main constituent members. Under recent demands for high efficiency and light weight of production apparatuses for semiconductor and liquid crystal, however, there has been pointed out following problems in constituent members made from stainless steel: insufficient in thermal conductivity, resulting in slow start-up in operation; heavy in its size, causing the apparatuses to be heavy as a whole. Besides, there have been occurred another problem since heavy metals such as Ni, Cr and the like included in stainless steel have a chance to be released to an environmental atmosphere so as to work as a contaminant source and thereby, deteriorate qualities of a semiconductor product and a liquid crystal product.
For the reason, aluminum alloy light in weight and high thermal conductivity has rapidly been increased in use, substituting stainless steel. Among various kinds of aluminum alloys, for example, JIS 3003 Al alloy including Mn: 1.0 to 1.5% , Cu: 0.05 to 0.20% and the like; JIS 5052 Al alloy including Mg: 2.2 to 2.8%, Cr.: 0.15 to 0.35% and the like; JIS 6061 Al alloy including Cu: 0.15 to 0.40%, Mg: 0.8 to 1.2%, Cr: 0.04 to 0.35% and the like are generally used. However, surfaces of such Al alloys are not good in resistance to corrosion caused by the above described corrosive gases and plasmas. Accordingly, it is indispensable to improve anti-corrosiveness of the Al alloys to the gases and plasmas in order for the Al alloys to be adopted as structural material of production apparatuses for semiconductor and liquid crystal. In order to improve the anti-corrosiveness, some treatment on an Al alloy surface is the most effective means.
Therefore, a technique has been proposed in the publication of Examined Japanese Patent Application No. Hei 5-53870, in which an anodic oxidation coating of A1203 excellent in anti-corrosiveness is formed on a surface of the above described Al alloys in order to increase anti-corrosiveness to the gas and plasma of the main constituent members of a vacuum chamber and the like. However, the anodic oxidation coating does not always satisfy requirements for anti-corrosiveness in all kinds of environments in which the main constituent members of a production apparatus for semiconductor are placed since a film quality of the anodic oxidation coating shows a largely different degree of anti-corrosiveness to gas or plasma according to environmental conditions.
For such a reason, there have been proposed various! methods to further improve a quality of an anodic oxidation coating in order to increase anti-corrosiveness of such Al alloys as materials of constituent members used in a semiconductor production apparatus. For example, in the publication of Unexamined Japanese Patent Application No. Hei 8-144088, a proposal is such that in formation of an anodic oxidation coating, an initial voltage for anodic oxidation is higher than a final voltage. Further, a proposal has been made in the Unexamined Japanese Patent Application No. Hei 8-144089, in which anodic oxidation is performed in a solution including a phosphate ion and a sulfate ion and a total opening area of pores on an anodic oxidation coating surface is adjusted in a specific range. Still further, other proposals appear in the publications in the Unexamined Japanese Patent Application Nos. Hei 8-260195 and Hei 8-260196, which disclose techniques in which a porous anodic oxidation coating is first formed and then, a coating by non-porous anodic oxidation is overlapped.
Any of such conventional techniques relating to anodic oxidation, as shown in FIG. 1, has a fundamental feature that recesses each called a pore 3 are started to be formed on a surface of a base material Al alloy 1 on start of electrolysis, continuing to be formed in progress of the oxidation and thereby, there is formed an anodic oxidation coating 6 comprising a porous layer 4 constructed of cells 2 that grows along the depth direction of the Al alloy 1 and a barrier layer 5. Since the barrier layer 5 has no gas permeability, gas or plasma is prevented from being put into contact with Al alloy. In the publication of Unexamined Japanese Patent Application No. Hei 8-193295 or the like, in order to further increase anti-corrosiveness to a plasma of such double-structured anodic oxidation coating, diameters of pores and cells on the surface side of the porous layer 4 have been proposed so as to be formed as small as possible.
An anodic oxidation coating such that the coating is constructed of the porous layer and barrier layer and diameters of pores and cells on the surface side of the porous layer 4 are formed as small as possible is sure to be excellent in anti-corrosiveness to gas and plasma. However, recent production conditions for semiconductor and liquid crystal have been very severe corresponding to a recent trend toward high efficiency and a large-size scale and gas and plasma related conditions are also severer due to transition toward a high concentration, a high density and high temperature. Accordingly, in recent years, structural materials of a reaction chamber and those of internal constituent members thereof have been required to possess anti-corrosiveness to the increasingly more severe corrosive gases and plasmas including halogen elements such as Cl, F, Br and the like, and elements such as O, N, H, B, S, C and the like, singly or in combination.
For example, evaluation of anti-corrosiveness to a halogen gas and a plasma appeared in the publication of the Unexamined Japanese Patent Application No. Hei 8-193295 is such as, for anti-corrosiveness to halogen gas, no corrosion under test conditions of 300xc2x0 C.xc3x974 hr in 5% Cl2xe2x80x94Ar and for anti-corrosiveness to plasma, 2 xcexcm or less in etching depth under test conditions of Cl2 plasma exposure for 90 min. On the other hand, anti-corrosiveness criteria required for structural materials of production apparatuses for semiconductor and liquid crystal with high efficiency are such as, for anti-corrosiveness to halogen, no corrosion after two time repetition of exposure to 5% Cl2 containing Ar gas at 400xc2x0 C. for 60 min and in addition, adhesiveness with no separation of a ceramic coating from an anodic oxidation coating in a tape separation test on the same sample. Further, for anti-corrosiveness to plasma, 1 xcexcm or less in etching depth after repetition of four time of exposure to Cl2 plasma for 60 min and to CF4 plasma for 30 min combined. An anodic oxidation coating obtained only by the above described treatment does not meet such severer requirements for anti-corrosiveness to the gases and plasmas.
On the other hand, in addition to the anodic oxidation coating, as materials excellent in anti-corrosiveness to the corrosive gas and plasma, there are available coatings of ceramic such as oxide (Al2O3), nitride (AlN), carbonitride (SiCN, AlCN), boride (TiB2), Silicide (MoSi2) and the like. There have sporadically been proposed examples in the publications of Examined Japanese Patent Application Nos. Hei 5-53872 and Hei 5-53871, in which the ceramic coatings are directly applied on an Al alloy surface by arc ion plating, sputtering, thermal spraying, CVD or the like. While the ceramic coatings are, however, without doubt excellent in anti-corrosiveness to halogen and plasma, it does not satisfy the recent severer requirements as in the case of the anodic oxidation coatings.
Therefore, such facts reveal that only individual improvements of an anodic oxidation coating and a ceramic coating have limitations to meet the anti-corrosiveness requirements. In order to satisfy the requirements for anti-corrosiveness to the gas and plasma, it is necessary that a concept of a composite coating is introduced and the ceramic coating is overlapped on the anodic oxidation coating to form a composite coating structure.
However, where a ceramic coating is overlapped on an anodic oxidation coating, a special problem arises in which adhesiveness between an anodic oxidation coating and a ceramic coating is poor. In particular, according to process conditions of production of semiconductor and liquid crystal, the constituent members of production apparatuses for semiconductor and liquid crystal in operation are subjected not only to the environment of a comparatively low temperature of 100xc2x0 C. or lower, but also to the severe working environments in which heat cycles (repetitions of rise and fall in working temperature) in a temperature range of 200 to 450xc2x0 C. Accordingly, the aforesaid constituent members require non-separable adhesiveness between an anodic oxidation coating and an Al alloy base material and between an anodic oxidation coating and a ceramic coating, against conditions not only in a range from room temperature to 100xc2x0 C., but also in high temperature heat cycles, and additionally in the corrosive environments of the gas and plasma, wherein a sample receives a halogen anti-corrosive test.
Therefore, in order to successfully stack a ceramic coating on an anodic oxidation coating, it is necessary to retain the adhesiveness even in the high temperature heat cycles and under the corrosive environment. Such composite coating has not been achieved in prior art, nor provided for practical use, if successful in a laboratory stage. In the publications of Examined Japanese Patent Application Nos. Hei 5-53782, Hei 5-53871, there have actually been disclosed a ceramic coating stacked directly on an Al alloy surface. The reason why is estimated that, as a decisive factor, adhesiveness between an anodic oxidation coating and a ceramic coating cannot be retained under conditions of the high temperature heat cycles and the corrosive environment and therefore, a function and an effect of anti-corrosiveness to the corrosive gas and plasma cannot be exerted.
The present invention has been made taking such circumstances into consideration and it is accordingly an object of the present invention to provide Al alloy with comprehensive anti-corrosiveness to gas and plasma, which has a composite-structured coating thereon of an anodic oxidation coating and a ceramic coating both excellent in anti-corrosiveness to the gas and the plasma, and whose composite-structured coating is improved especially on adhesiveness between the anodic oxidation coating and the ceramic coating in heat cycles in the range from room temperature (or in a some case, lower than room temperature) to a high temperature and under a corrosive environment.
In order to achieve the object, the features of the present invention is that aluminum alloy of the present invention is aluminum alloy on whose surface an anodic oxidation coating and a ceramic coating are stacked in the order, wherein the anodic oxidation coating contains one or more elements selected from the group consisting of C, N, P, F, B and S each at a content of 0.1% or more and the ceramic coating is made of one or more selected from the group of oxide, nitride, carbonitride, boride and silicide, and/or one or more selected from the group consisting of carbides expressed by MC (wherein M is any of Sif Ti, Zr, Hf, V, Nb, Ta, and Mo) , carbides expressed by M2C (wherein M is any of V, Ta, Mo and W) , carbides expressed by M3C (wherein M is any of Mn, Fe, Co and Ni) and carbides expressed by M3C2 (wherein M is Cr). (Percentage of elements in this specification is mass %.)
In the publication of Unexamined Japanese Patent Application No. Hei 8-193295 as well, it is disclosed that when an anodic oxidation coating contains two or more elements selected from the group consisting of C, S, N, P, F and B, the anodic oxidation coating excellent in anti-corrosiveness to gas and plasma can be obtained. However, in the publication, there are no disclosure that a ceramic coating is further stacked on the anodic oxidation coating that contains such an element and adhesiveness between the anodic oxidation coating that contains such an element and the ceramic coating is excellent especially under conditions of the high temperature heat cycles and an corrosive environment. Further, anti-corrosiveness to gas and plasma is low in degree compared with the present invention as described above.
According to findings by the inventors of the present invention, an ordinary hard anodic oxidation coating formed from an aqueous solution of sulfuric acid as a main component, which process has conventionally been conducted, contains only S of the above described elements. The ordinary hard anodic oxidation coating with only S contained cannot enjoy an effect on improvement of adhesiveness between an anodic oxidation coating and a ceramic coating under conditions of the high temperature heat cycles and an corrosive environment.
However, according to a study of the inventors of the present invention, adhesiveness of the ceramic coating to the anodic oxidation coating can be secured by a physical anchor effect even in a case where the anodic oxidation coating contains only S if a roughness of the hard anodic oxidation coating is increased sufficiently through roughening an Al alloy surface, in a more concrete manner of description, if an average roughness Ra of the Al alloy surface or an anodic oxidation coating is 0.3 xcexcm or more, preferably 0.5 xcexcm or more, or more preferably 0.8 xcexcm or more, in contrast with a surface state of the ordinary hard anodic oxidation coating, that is the hard anodic oxidation coating in the case where surface roughening intentionally or positively is not performed on the Al alloy or the anodic oxidation coating. That is, in a case where an average roughness of a surface of Al alloy or an anodic oxidation coating is adjusted 0.3 xcexcm or more in Ra, an improving effect on adhesiveness even only with S contained can be exerted.
In the present invention, one or more elements selected from the group consisting of C, N, P, F, B and S are each included at 0.1% or more (provided that in a case of S, an average roughness Ra of an Al alloy or an anodic oxidation coating is adjusted to be 0.3 xcexcm or more) and thereby, adhesiveness between the anodic oxidation coating and the ceramic coating under conditions of the high temperature heat cycles and an corrosive environment is improved by a great margin. Further, when S is included in a composite manner in addition to one or more elements selected from the group consisting of C, N, P, F and B, an improving effect on adhesiveness that cannot be obtained with S singly used can be achieved by a composite effect of the other element or elements and S as described later.
Further, by improvement of adhesiveness between the anodic oxidation coating and the ceramic coating, a composite coating structure is enabling in which the anodic oxidation coating is formed on a surface of the Al alloy and a ceramic coating is stacked on the anodic oxidation coating and anticorrosiveness to plasma is guaranteed by the ceramic coating and anti-corrosiveness to halogen gas is guaranteed by the anodic oxidation coating.